Sizers and stretch reducers, which are the most common types of reducers, are normally constituted by a plurality of pipe rolling stands (such as 8-28 stands) installed in tandem, with each stand being equipped with grooved rolls of the two-roll or three-roll type. For example, in the three-roll reducing method using grooved rolls of the three-roll type, the grooved rolls are installed in each stand so that the grooved rolls in adjoining stands have a phase angle (phase difference) of 60 degrees with respect to each other. A pipe undergoes rolling by passing through the groove (pass) formed from the grooved rolls each having an elliptical groove shape with no tool being inserted into the interior of the pipe and in the case of a stretch reducer with tension being applied to the pipe between adjoining stands. The rolling greatly decreases the outer diameter of the pipe, with the wall thickness generally increasing in the case of a sizer and generally decreasing in the case of a stretch reducer. In the two-roll reducing method using grooved rolls of the two-roll type in each stand, there is a phase difference of 90 degrees between the grooved rolls of adjoining stands.
Because the roll grooves each have an elliptical shape, a pipe which is subjected to reducing by grooved rolls is strongly deformed in the central portion in the axial direction of each grooved roll (referred to as the groove bottom zone), and the rolling force decreases towards both end portions of the groove (the end portions of the flange zones on both sides of the groove bottom zone). Since a tool is not present in the interior of the pipe, when the pipe is subjected to several passes through rolling stands, so-called polygonization takes place. Polygonization is a phenomenon in which the transverse cross-sectional shape of the inner surface of a pipe becomes hexagonal (or tetragonal in a two-roll reducing method).
A pipe which has developed polygonization has a polygonal transverse cross-sectional shape on its inner surface, but the outer surface of the pipe which has been finished in the reducer is nearly circular. Therefore, the wall thickness of the pipe exhibits wall thickness variations (thickness deviations) in which the wall thickness periodically increases or decreases in the circumferential direction (three or six times in the case of three-roll reducing). Polygonization is known to occur particularly easily when carrying out rolling with a reducer of a pipe having an intermediate or large wall thickness in which the ratio of the finished wall thickness to the finished outer diameter is 8% or greater.
As described in below-listed Patent Documents 1 and 2, the degree of this polygonization varies with the rectangularity of a roll groove, which is expressed by the ratio (CLE/CLG) of the distance CLE from the edge portion entrance surface to the roll exit surface with respect to the distance CLG from the roll groove bottom entrance surface to the roll exit surface. A known countermeasure against polygonization is the rectangularity design method in which the rectangularity is suitably selected.
Patent Document 3 discloses suppressing polygonization by suitably setting the amount of working in each stand of a plurality of roll stands having grooved rolls. Patent Document 4 discloses minimizing polygonization by setting the rotational speed of grooved rolls in each stand to a suitable value so that the overall elongation of a rolled pipe is made uniform by controlling the rotational speed of drive motors which rotationally drive the grooved rolls in each stand of a stretch reducer.
Patent Document 5 discloses suppressing polygonization by water cooling of portions of a pipe during reducing of the pipe. Patent Documents 6 and 7 disclose suppressing polygonization during sizing rolling by adjusting the roll position in each stand. Patent Documents 8-10 disclose suppressing polygonization by suitably setting the phase angle of the rolls in each stand during reducing of a pipe.
Patent Document 1: JP H07-314013 A1
Patent Document 2: JP H08-19808 A1
Patent Document 3: JP H11-151506 A1
Patent Document 4: JP 2001-71012 A1
Patent Document 5: JP 2001-129603 A1
Patent Document 6: JP 2000-158015 A1
Patent Document 7: JP 2000-334504 A1
Patent Document 8: JP 2005-46874 A1
Patent Document 9: JP 2005-305447 A1
Patent Document 10: JP 2005-169466 A1
However, with the techniques disclosed in Patent Documents 1-3, the amount of polygonization, which varies in accordance with conditions such as the wall thickness and tension of a pipe, cannot always be suppressed to a constant range under all conditions. Similarly, the techniques disclosed in Patent Documents 4, 6, and 7 can only slightly decrease the amount of polygonization which occurs, and they cannot suppress polygonization to a fixed range regardless of variations in conditions.
The technique disclosed in Patent Document 5 is premised on variation in the heating of a pipe being the main cause of polygonization. However, if the temperature of a pipe during rolling is locally decreased by water cooling, cooling water unavoidably splashes or flows to portions other than the desired portion, and it is extremely difficult to control the temperature only of a specific portion of a pipe. Accordingly, it is thought to be difficult to stably suppress polygonization with this technique.
In order to carry out the techniques disclosed in Patent Documents 8-10, it is necessary to adjust the phase angle of the grooved rolls of each stand. For this purpose, it is necessary to change the conditions for reducing, and the rectangularity of a groove, which is a direct cause of polygonization, fluctuates, so this method cannot stably suppress polygonization.